Free-radical initiators, particularly peroxidic initiators such as benzoyl peroxide, are commonly used as the primary initiator, or catalyst, in the suspension polymerization of vinyl aromatic monomers such as styrene. These initiators, being predominantly oil soluble and water insoluble, are believed to react within the monomer droplets in suspension to cause the polymerization in the following manner: EQU ROOR .sup.Heat 2RO.degree. EQU ro.degree. + m.fwdarw.rom.degree. .sup.m romm.degree. etc.
Grim Pat. No. 2,673,194, describes the suspension polymerization of vinyl aromatic monomers, whereby an oily monomer is suspended as droplets in an aqueous medium and polymer beads or particles are produced by the use of an oil-soluble polymerization catalyst, such as benzoyl peroxide. The suspending system of the Grim patent is comprised of finely divided, difficultly water-soluble, inorganic phosphates, and an anionic surface-active agent which serves as a modifier. As used in this art, compounds which act to increase the ability of the finely divided phosphate dispersant to stabilize suspensions are termed modifiers. The modifiers increase the ability of the phosphate dispersant to maintain stable suspensions with a greater proportion of monomer and/or polymer in the suspension polymerization medium. Among the modifiers listed by Grim are: sodium dodecylbenzene sulfonate, sodium tetradecyl sulfate, potassium stearate, and long chain alkyl sulfonates.
The suspension systems such as that of Grim produce polymer beads having a broad particle size distribution. The individual beads produced in suspension may range in diameter from less than 300 microns to above 2,000 microns with the average bead diameter being dependent upon the amount of suspending agent and modifier present in the system. The average bead diameter can be controlled to some extent by varying the parameters of the system such as the ratio of suspending agent to modifier or the ratio of suspending agent and modifier to monomer. Although the average diameter size is changed through such variations, nevertheless, beads will be produced whose particle size distribution is broad.
For commercial reasons, it is highly desirable in this art to control the average particle size range within relatively narrow limits. This is especially true where the polymer beads are to be impregnated with a volatile expanding agent to produce polymer beads which will, upon heating, expand, e.g. in a mold, to fill the mold cavity and produce fused, expanded polymer articles.
One of the major uses of expandable polymer beads, particularly polystyrene beads, which are produced commercially by suspension polymerization, is in insulation applications such as insulation board. In manufacturing insulation board, it is common to mold a large (e.g. 4 .times. 8 .times. 20 feet) billet of expanded polystyrene and subsequently cut the billet into slabs 1 to 2 inches thick for ultimate use as an insulation board. In preparing the large billets from expandable polymer beads, the beads are first pre-expanded to form non-fused, partially expanded beads having a bulk density of from 0.8-1.2 pounds per cubic foot. The partially expanded beads are then charged to the billet mold, and heat, usually in the form of steam, is applied to fully expand the beads whereby they fill the mold, fuse, and form the billet.
In the billet-molding application, it is especially critical that the expandable polymer beads be relatively large and substantially spherical.
If the beads are too small, then the expandable beads at the outer surface of the billet mold will fuse too soon, thereby excluding steam from the center of the mold. The resulting billet, therefore, has a center of unfused, expanded beads which is, of course, highly undesirable since any unfused portion of the billet is useless.
Hohenstein et al, in U.S. Pat. No. 2,652,392, reduced the amount of small particle-size beads formed by adding less than 0.01 percent based on monomer of a water-soluble persulfate as modifier for the calcium phosphate stabilizer.
I have shown in my U.S. Pat. No. 3,631,014, that narrow distribution of bead sizes larger than 300 microns (45 mesh) in diameter can be produced by the addition of at least 0.0003 percent by weight based on monomer of sodium bisulfite as modifier for the phosphate.
I have also shown in my U.S. Pat. No. 3,649,610, that narrow distribution of bead sizes larger than 750 microns (25 mesh) in diameter can be produced by the addition of at least 0.01 percent by weight based on monomer of certain terminal vicinal hydroxy-keto compounds as modifier for the phosphate.
I have also shown in my U.S. Pat. No. 3,755,282, that narrow distribution of bead sizes larger than 1200 microns (16 mesh) in diameter can be produced by the addition of at least 0.0001 percent by weight based on monomer of .alpha., .beta.-unsaturated carboxylic acids as modifier for the phosphate.